The invention relates to controlling the ion population in a mass analyzer.
Ion storage type mass analyzers, such as RF quadrupole ion trap, ICR (Ion Cyclotron Resonance), orbitrap, and FTICR (Fourier Transform Ion Cyclotron Resonance) mass analyzers, function by transferring generated ions via an ion optical means to the storage/trapping cells on the mass analyzer, where the ions are then analyzed. One of the major factors that limit the mass resolution, mass accuracy and the reproducibility in such devices is space charge, which can alter the storage, trapping conditions, or ability to mass analyze of an ICR or ion trap, from one experiment to the next, and consequently vary the results attained.
Similarly, in operation of a Time of Flight (TOF) system, or a hybrid TOF mass spectrometer, such as a Trap-TOF, the operator typically attempts to deliver as high an absolute ion rate as possible to the TOF to maximize sensitivity, but not so high as to saturate the detection system. When dealing with internal mass standards for high mass accuracy measurements, this problem is further compounded by the need to match closely the relative intensities of the internal standard and the analytes of interest.
Space charge effects arise from the influence of the electric fields of trapped ions upon each other. The combined or bulk charge of the final population of ions causes shifts in frequency and therefore m/z. At very high levels of space charge, the obtainable resolution will deteriorate and peaks close in frequency (m/z) can at least partially coalesce. A significant scan to scan variation in the magnitude of the space charge effect arises from differences in trapped ion density, caused by changes in the number of ions within the cell from one ionization/ion injection event to the next. Unless space charge is either taken into account or regulated, high mass accuracy, precision mass and intensity measurements can not be reliably achieved.
In a uniform magnetic field and in the absence of any other forces on the ion, the angular frequency of motion of an ion is a simple function of the ion charge, the ion mass, and the magnetic field strength:ω=qB/mwhere ω=angular frequency, q=ion charge, B=magnetic field strength, and m=ion mass. This simplified equation ignores the effects of electric fields on the frequency of the ion. As described by Francl et al., “Experimental Determination of the Effects of Space Charge on Ion Cyclotron Resonance Frequencies” Int. J. Mass Spectrom. Ion Processes, 54, 1983 p. 189–199, which is incorporated by reference herein, the cyclotron frequency of the ion in an ICR cell can be approximately described by:ω=qB/m−2αV/a2B−qρGi/ε0Bwhere α is a cell geometry constant, V is the trapping voltage, a is the cell diameter, ρ is the ion density, Gi is an ion cloud geometry constant, and ε0 is the permittivity of free space.
Hence, if the ion population in a FTICR is allowed to vary, the measured peak positions will move as a result of the interaction of the ions with the electrostatic fields of the other ions in addition to the fields of the cell and magnet. This has been a relatively minor problem, resulting in mass shifts of a few 10's of ppm. However, as analytical requirements have progressed, it now has become desirable to obtain mass accuracies in the single ppm range.
One way to improve the reproducibility of results, the mass resolution and accuracy in ion storage type devices is to control the ion population that is stored/trapped, and subsequently analyzed in the mass analyzer.